In U.S. Pat. No. 8,529,014, the contents of which are incorporated herein by reference, there was described a digital inkjet press comprised of a plurality of fixed monochrome printheads, which are aligned with each other in a media feed direction. Printing systems of this type are increasingly being used for relatively short print runs as an alternative to traditional analog printing presses. Digital printing advantageously obviates the creation of unique offset printing plates for each print job and reduces overall printing costs, especially in relatively short print runs.
In the printing system described in U.S. Pat. No. 8,529,014, there are provided five monochrome printheads, which can be plumbed with any required ink combination. As used herein, the term ‘blade’ is used to refer to a monochrome printhead and its associated ink delivery system. For pure monochrome printing at high speeds, the five blades may be configured as KKKKK; for typical color printing, the five blades may be configured as CMYKK—the two black channels enhancing black optical density; for customers with particular color requirements, the five blades may be configured as CMYKS, where S is a spot color, such as orange.
Customers are generally sensitive to the appearance of spot colors in, for example, labels and marketing materials. For example, a customer may have a brand association with a particular color which needs to be reproduced consistently in materials generated by both analog and digital printing techniques. Consistent color reproduction involves not only matching color, but also matching the perceived vividness of the color. However, digital inkjet presses, such as the printing system described in U.S. Pat. No. 8,529,014, have limitations, which make it difficult to provide a spot color that matches a corresponding analog spot color. Firstly, inkjet ink formulations have a limit on the maximum colorant loading useable in inks. With high colorant loadings, inkjet nozzles, as well as associated ink delivery and maintenance systems, can be become clogged with ink. Secondly, digital inkjet presses may have a limited and fixed number of blades, as defined by the architecture of the printing system. It would be convenient to simply add another spot color blade to compensate for the relatively lower colorant loading in inkjet inks; however, in the fixed five blade printing system described in U.S. Pat. No. 8,529,014, an additional spot color blade (e.g. CMYSS) would undesirably restrict the overall color gamut of the printer. Alternatively, a higher density of spot color may be achieved with slower print speeds, but slower printing speeds are obviously disadvantageous from most customers' standpoint.
It would therefore be desirable to provide a method of printing from a digital inkjet press, which improves on existing approaches to reproduction of spot colors, such as orange.